Rare earth element recovery method

ABSTRACT

Provided is a method of recovering rare-earth elements, including causing rare-earth elements particularly including Nd and Dy to leach efficiently from a raw material for leaching which contains the rare-earth elements, and separating and recovering the rare-earth elements. The method of recovering rare-earth elements includes: a leaching step including performing leaching treatment of rare-earth elements in which an acidic slurry of a raw material for leaching which contains the rare-earth elements is held under a predetermined condition, and then subjecting the slurry obtained after the leaching treatment to solid-liquid separation, yielding a leachate containing the rare-earth elements; and a separation step of separating and recovering the rare-earth elements from the yielded leachate, in which: the raw material for leaching contains Ca as CaO at a ratio of 4 to 15 mass % and Ti as TiO 2  at a ratio of 2 to 13 mass % in a solid component (S); an acid aqueous solution is an acid aqueous solution of hydrochloric acid and/or nitric acid; and the leaching treatment performed in the leaching step is digestion or maceration which is performed under the heating and pressurizing conditions of a temperature of 160 to 300° C. and a pressure of 0.65 to 10 MPa, and the rare-earth elements are caused to leach together with Ca in the leaching step.

TECHNICAL FIELD

The present invention relates to a method of recovering rare-earthelements involving causing rare-earth elements, in particular,rare-earth elements including Nd and Dy, which are highly useful asmaterials for an Nd—Fe—B-based permanent magnet, to leach from a rawmaterial for leaching which contains rare-earth elements, and separatingand recovering the rare-earth elements, and more particularly, to amethod of recovering rare-earth elements involving causing rare-earthelements to leach efficiently together with Ca from a raw material forleaching which contains Ca and Ti, and separating and recovering therare-earth elements.

BACKGROUND ART

Rare-earth elements have been widely used in applications such as aphosphor, a magnetic substance, an abrasive, and a catalyst.Particularly in the magnetic substance, the use of the rare-earthelements as materials for a permanent magnet has been rapidly expandingbecause a magnet having a large maximum energy product and a largeresidual magnetic flux density can be obtained by adding the rare-earthelements to transition elements. For example, Patent Literature 1discloses materials for an Nd—Fe—B-based permanent magnet having anexcellent maximum energy product and an excellent residual magnetic fluxdensity. In addition, Patent Literature 2 discloses a technology forimproving the thermal stability of magnetic characteristics, which is adefect of the Nd—Fe—B-based permanent magnet, by substituting part of Ndwith Dy in the permanent magnet.

For example, ores such as monazite, bastnaesite, xenotime, and ionadsorption clay mineral are used as raw materials for such rare-earthelements. The rare-earth elements are caused to leach from any of theseores by using an acid aqueous solution, for example, an aqueous solutionof a mineral acid such as sulfuric acid, and the rare-earth elements areseparated and collected from the resultant leachate. However, these oreresources are unevenly distributed on the earth, and the abundance ofeach element in the rare-earth elements significantly varies for eachore. In particular, there are very few mines in which ores containingheavy rare-earth elements having atomic numbers of 64 to 71 and havinghigh mine profitability can be mined, and it is concerned that thedepletion of the resources of Dy, which is in especially high demand,may occur.

On the other hand, the rare-earth elements are also contained inbauxite, which exists as a resource abundantly and which is an oreresource of aluminum. It is known that the rare-earth elements arecaused to dissolve from bauxite and are then separated and recovered(see, for example, paragraph 0003 in Patent Literature 3). Further, itis also known that rare-earth elements are caused to leach by using, asa raw material, a solid residue produced as a by-product in a Bayerprocess and remaining after the collection of aluminum in the productionof aluminum from the bauxite through the steps of the Bayer process anda Hall-Héroult process (The solid residue is hereinafter referred to as“bauxite residue”. A bauxite residue containing Fe₂O₃ as a maincomponent has a red color and is generally called “red mud”.) and arethen separated and recovered (Patent Literature 4).

In addition, the rare-earth elements are stable in an alkaline aqueoussolution by taking the forms of compounds such as oxides and hydroxides,and the compounds do not react with a solution of sodium hydroxide evenwhen the solution is heated and pressurized. Thus, in the bauxiteresidue, the rare-earth elements are to be concentrated to the extentcorresponding to the amount of the aluminum component caused to leachwith the solution of sodium hydroxide in the Bayer process describedabove. According to studies of the inventors of the present invention,the bauxite residue contains rare-earth elements about three times onthe average in comparison to the content of rare-earth elements in thebauxite. Further, the bauxite residue is an industrial waste which isproduced in the production of aluminum from bauxite, and is stablyproduced as a by-product in the production of aluminum, and hence can beeasily obtained. Therefore, the bauxite residue is expected to be usedas a raw material for rare-earth elements.

However, detailed examination of Patent Literature 4 above has revealedthat, as described in Examples 1 and 2 thereof, a bauxite residuecontaining, in a dried state, 52.0% of Fe₂O₃, 6.5% of TiO₂, 18.0% ofignition loss, 12.9% of Al₂O₃, 2.4% of SiO₂, 1.6% of Na₂O, 5.0% of CaO,and 0.6% of P₂O₅ is used as a raw material, and a leaching operation(leaching, or digestion or maceration) is repeated two or three times at10 to 70° C. by using a sulfurous acid solution having a high pH valueand a sulfurous acid solution having a low pH value to adjust the finalph value to 1.35 to 2.4. Accordingly, rare-earth elements are caused toleach while keeping the dissolution of Fe and Ti contained in thebauxite residue at a low level, and the rare-earth elements are thenseparated and recovered by using a solvent extraction method. In thiscase, although 50 to 85% of the content of Y in the bauxite residue arecaused to leach and the leaching ratio of Dy is not described, in aleaching time of 20 minutes, which is considered to be preferred toalmost saturate the leaching amount of the rare-earth elements withoutcontinuously increasing the leaching amount of Fe, the leaching ratio ofNd is lower than that of Y and is only about 58% (see the descriptionson lines 32 to 36 in column 7, Tables 1 to 3, and FIG. 2 in PatentLiterature 4).

That is, the technology described in Examples 1 and 2 of PatentLiterature 4 involves repeating the leaching operation two or threetimes, and hence, as the amount of a leachate increases, the cost of theleaching step increases at the time of causing rare-earth elements toleach from a bauxite residue because, for example, it is required torepeat solid-liquid separation two or three times. Moreover, theliquid-solid ratios at the time of the leaching operations are set to4:1 and 10:1 in digestion or maceration carried out twice in Example 1(see Table 1) and set to 4:1 and 8:1 in digestion or maceration carriedout twice in Example 2 (see Table 3). Accordingly, the amount of aleachate becomes 14 times or 12 times the amount of the bauxite residueserving as a raw material. Thus, this technology has a problem in that asolvent extraction apparatus in the separation step of separating andrecovering rare-earth elements from the leachate by the solventextraction method is increased in size and the cost thereof is alsoincreased.

By the way, the inventors of the present invention used 0.102 kg of abauxite residue having the same composition as that of the bauxiteresidue used in Examples to be described below, and followed theexperiment in Example 1 of Patent Literature 4, which involves using anaqueous solution of sulfurous acid as an acid aqueous solution andrepeating the same leaching operation three times under the conditionsof a liquid-solid ratio (L/S) of 5.0, a temperature of 30° C., apressure of 0.1 MPa, and a time of 15 minutes. The results are as shownin Table 1. In the first leaching operation, the leaching ratio of Ymerely reached 5 mass % or less, and the total leaching ratio of Yadditionally including the leaching ratios of the second and thirdleaching operations was 52 mass %. However, the leaching ratios of Ndand Dy merely reached 41 mass % and 43 mass %, respectively, which weremerely even lower values in comparison to the leaching ratio of Y.

TABLE 1 Usage of bauxite residue kg 0.102 First Kind of acid H₂SO₃leaching Liquid-solid ratio 5.0 Leaching Temperature ° C. 30 conditionspH After completion 3.27 of leaching Time Minutes 15 Second Kind of acidH₂SO₃ leaching Liquid-solid ratio 5.0 Leaching Temperature ° C. 30conditions pH Initial stage 2.05 of leaching After completion 3.20 ofleaching Time Minutes 15 Third Kind of acid H₂SO₃ leaching Liquid-solidratio 5.0 Leaching Temperature ° C. 30 conditions pH Initial stage 1.21of leaching After completion 1.82 of leaching Time Minutes 15 pH valueInitial stage of leaching 3.3 After leaching 1.2 Leaching Y 52 ratio Nd41 (mass %) Dy 43 Ca 88 Al 40 Si 99 Ti 0.3 Fe 0.2

CITATION LIST Patent Literature

-   [PTL 1] JP 59-046008 A-   [PTL 2] JP 62-165305 A-   [PTL 3] JP 09-184028 A-   [PTL 4] U.S. Pat. No. 5,030,424 A

SUMMARY OF INVENTION Technical Problem

In view of the foregoing, the inventors of the present invention havemade studies on the causes of the low leaching ratio of rare-earthelements, in particular, Nd and Dy in the leaching operation ofrare-earth elements contained in the bauxite residue, and have reachedthe following conclusion.

That is, when aluminum is produced by a Bayer process using bauxite as araw material, the bauxite is mixed with an aqueous solution of sodiumhydroxide, the mixture is heated and pressurized, thereby causing itsaluminum component to dissolve as aluminate ions, the resultant eluatecontaining the aluminum component is cooled, thereby causing thealuminate ions to precipitate as aluminum hydroxide, and the aluminumhydroxide is calcined to collect aluminum oxide. In the Bayer process,CaO is often added in order to recover, as sodium hydroxide, a sodiumcompound produced by a reaction between a component in the bauxite andthe aqueous solution of sodium hydroxide and to remove impurities suchas Si and P, and hence CaO is usually contained in the bauxite residueat 4 to 15 mass %.

Then, in the Bayer process, when CaO is added into a sodium aluminatesolution having as high a temperature as 160° C. or more, Ti containedin the bauxite reacts with Ca added as CaO, producing calcium titanate(CaTiO₃), which forms a crystal having a perovskite (ABX3)-typestructure. Further, when the produced calcium titanate forms a crystal,parts of rare-earth elements such as Nd and Dy contained in the bauxiteare incorporated into the crystal. Besides, the crystal formed of thecalcium titanate (CaTiO₃) and having a perovskite (ABX3)-type structuredoes not easily dissolve in a mineral acid at less than 160° C., andhence the inventors of the present invention have consequently concludedthat it is difficult to increase its leaching ratio by performing ausual leaching operation.

In view of the foregoing, the inventors of the present invention havefurther studied intensively a method involving: causing rare-earthelements to leach efficiently from a raw material for leaching whichcontains compounds such as calcium titanate, which forms the crystalhaving a perovskite (ABX3)-type structure as described above, therare-earth elements including rare-earth elements such as Nd and Dyincorporated in the crystal; and separating and recovering therare-earth elements from the resultant leachate. As a result, theinventors have found that, surprisingly, the crystal having a perovskite(ABX3)-type structure can be easily dissolved by performing digestion ormaceration with a particular acid aqueous solution under particularheating and pressurizing conditions, and hence it is possible to notonly cause, as a matter of course, the rare-earth elements notincorporated in the crystal to leach easily, but also cause therare-earth elements incorporated in the crystal to leach easily. Thus,the present invention has been completed.

Therefore, an object of the present invention is to provide a method ofrecovering rare-earth elements involving causing rare-earth elements, inparticular, rare-earth elements including Nd and Dy to leach efficientlyfrom a raw material for leaching which contains the rare-earth elements,and separating and recovering the rare-earth elements.

Solution to Problem

That is, according to one embodiment of the present invention, there isprovided a method of recovering rare-earth elements, including:

a leaching step including preparing a slurry by adding water to a rawmaterial for leaching which contains rare-earth elements, followed bymixing, further adding an acid aqueous solution to the slurry, followedby mixing, to adjust a pH of the slurry, performing leaching treatmentin which the rare-earth elements in the raw material for leaching aretransferred into the acid aqueous solution while the resultant slurry isheld under a predetermined condition, and then subjecting the slurryobtained after the leaching treatment to solid-liquid separation,yielding a leachate containing the rare-earth elements; and

a separation step of separating and recovering the rare-earth elementsfrom the leachate yielded in the leaching step, in which:

the raw material for leaching includes Ca as CaO at a ratio of from 4 to15 mass % and Ti as TiO₂ at a ratio of from 2 to 13 mass % in a solidcomponent (S) obtained by drying the raw material for leaching underdrying conditions of 110° C. and 2 hours;

the acid aqueous solution includes an acid aqueous solution whichcontains hydrochloric acid and/or nitric acid and adjusts the pH to from0 to 2.7; and

the leaching treatment performed in the leaching step is digestion ormaceration which is performed under heating and pressurizing conditionsof a temperature of from 160 to 300° C. and a pressure of from 0.65 to10 MPa, and the rare-earth elements in the raw material for leaching arecaused to leach together with Ca in the leaching step.

In the present invention, it is desirable that the digestion ormaceration in the leaching step be performed until the dissolution ratioof Ca contained in the raw material for leaching reaches 90 mass % ormore, and consequently, rare-earth elements including Y and alsoincluding Nd and Dy, which are highly useful, can be recovered at ashigh a leaching ratio as more than 70 mass %.

Herein, in the present invention, the term “rare-earth elements” is usedto refer collectively to Y with an atomic number of 39 and La to Lu withatomic numbers of 57 to 71. According to the method of the presentinvention, Sc with an atomic number of 21 and Ac to Lr with atomicnumbers of 89 to 103 are caused to leach, but the present invention doesnot deny the possibility that these elements are caused to leach and areseparated and recovered.

In the present invention, the raw material for leaching which containsrare-earth elements is not particularly limited as long as the rawmaterial for leaching contains rare-earth elements such as Y, Nd, and Dyand contain Ca as CaO at a ratio of 4 to 15 mass % and Ti as TiO₂ at aratio of 2 to 13 mass %. The raw material for leaching which containsrare-earth elements is preferably a bauxite residue produced as aby-product in a Bayer process for causing an aluminum component to leachfrom bauxite by using an aqueous solution of sodium hydroxide, morepreferably a bauxite residue including rare-earth elements as oxidesthereof at a ratio of 500 to 10,000 ppm in a solid component (S)obtained by drying the bauxite residue under the drying conditions of110° C. and 2 hours. Such bauxite residues are produced as by-productsin a Bayer process for collecting an aluminum component from bauxite, inparticular, a Bayer process in which a sodium component is recovered assodium hydroxide and CaO is added to remove impurities such as Si and P,and hence the bauxite residues are easily available in a large quantity.

Herein, Ca and Ti in the bauxite residue are considered to form acrystal having a perovskite (ABX3)-type structure. In the crystal havinga perovskite (ABX3)-type structure, cations in the A site and anions inthe X site have almost the same size, and inside the cubic lattice ofthe crystal structured by the A site and the X site, cations each havinga smaller size than the cations in the A site are located in the B site.In the crystal having a perovskite (ABX3)-type structure, elements aredensely coordinated and are stable in a high pressure state. The size ofions in the A site and B site is allowable in the range of a tolerancefactor of t=0.75 to 1.1. Further, when oxygen is located in the X site,elements are chosen so that the valencies of the A site and B sitesatisfy the equation A+B=6. Thus, various elements can be dissolved inthe A site and B site as long as those elements satisfy the valenciesand tolerance factor. Each rare-earth element has a large ionic radiusand is trivalent, and hence it is considered that the rare-earth elementis dissolved as a pair with an Fe ion, which has a smaller ionic radiusand is trivalent. Note that the tolerance factor t is represented by thefollowing equation.

$t = \frac{\left( {{ionic}\mspace{14mu}{radius}\mspace{14mu}{at}\mspace{14mu} A\mspace{14mu}{site}} \right) + \left( {{ionic}\mspace{14mu}{radius}\mspace{14mu}{at}\mspace{14mu} X\mspace{14mu}{site}} \right)}{\sqrt{2 \times \left\{ {\left( {{ionic}\mspace{14mu}{radius}\mspace{14mu}{at}\mspace{14mu} B\mspace{14mu}{site}} \right) + \left( {{ionic}\mspace{14mu}{radius}\mspace{14mu}{at}\mspace{14mu} X\mspace{14mu}{site}} \right)} \right\}}}$

Advantageous Effects of Invention

According to one embodiment of the present invention, it is possible tocause rare-earth elements including not only Y but also Nd and Dy, whichare highly useful, to leach efficiently and easily, followed byseparation and recovery of the rare-earth elements, from a raw materialfor leaching which is, for example, a bauxite residue that is generatedas an industrial waste when aluminum is produced from bauxite, and whichincludes Ca as CaO at a ratio of 4 to 15 mass % and Ti as TiO₂ at aratio of 2 to 13 mass %. As a result, resources in a bauxite rawmaterial can be utilized effectively, and it is possible to eliminateconcerns such as the uneven distribution of raw material ores forrare-earth elements, the variation in the abundance of each element inthe rare-earth elements for each ore, and the depletion of the resourcesof rare-earth elements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart illustrating the removal of impurity elements ina leachate and the concentration of rare-earth elements in the leachateperformed by a two-stage solvent extraction method according to Example53 of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention are specificallydescribed by taking as an example a case where a raw material forleaching which contains rare-earth elements is a bauxite residue.

First, in a leaching step, an acid aqueous solution is added to thebauxite residue, followed by mixing, thereby preparing a slurry. Theacid aqueous solution to be used here is desirably an acid aqueoussolution containing hydrochloric acid and/or nitric acid, which does notform an insoluble compound with Ca in the bauxite residue even if theslurry is heated to 160° C. or more.

Further, it is desirable that the slurry to be prepared have aliquid-solid ratio (L/S) of a liquid component (L) and a solid component(S) of preferably 2 or more and 10 or less, more preferably 2 or moreand 10 or less, and have a pH value of preferably 0 or more and 2.7 orless, more preferably 0 or more and 2.5 or less. When the preparedslurry has a liquid-solid ratio (L/S) of less than 2, the leaching ratioof rare-earth elements reduces to an insufficient level, and theviscosity of the slurry increases, with the result that it is difficultto handle the slurry in the subsequent separation step. On the otherhand, even when the liquid-solid ratio (L/S) is adjusted to more than10, the leaching ratio of rare-earth elements is saturated and does notimprove, and, in addition, the amount of a leachate increases, with theresult that a load in the subsequent separation step is too high.Further, when the prepared slurry has a pH value of higher than 2.7, theleaching ratio of rare-earth elements reduces to an insufficient level.On the other hand, when the pH value is adjusted to less than 0, thedissolution amounts of Al and Fe increase, with the result that it isdifficult to separate rare-earth elements, and the acid and a pHadjuster to be described below are consumed more, causing an increase incost, which is not preferred.

Further, in the slurry prepared by adding the acid aqueous solution tothe bauxite residue, it is desirable to add an oxidizing agent at aratio of 0.1 to 1 equivalent weight, preferably at a ratio of 0.15 to0.4 equivalent weight, with respect to an Fe component in the bauxiteresidue, for the purpose of converting Fe²⁺ ions in the slurry which arederived from the bauxite residue to Fe³⁺ ions, thereby facilitating anoperation for causing Fe and Al to precipitate and be separated in thesubsequent separation step. The oxidizing agent to be added for thispurpose may be exemplified preferably by a hydrogen peroxide solutionand a perchloric acid aqueous solution, more preferably by a 30-mass %hydrogen peroxide solution and a 70-mass % perchloric acid aqueoussolution. When the addition amount of the oxidizing agent is less than0.1 equivalent weight, there arises a problem in that Fe²⁺ ions remainin the leachate even in the state of a high pH. In contrast, even whenthe addition amount of the oxidizing agent is more than 1 equivalentweight, there arises a problem in that the effect of the oxidizing agentremains unchanged, and hence the oxidizing agent excessively added isused wastefully.

Next, in the present invention, leaching treatment of rare-earthelements is performed while the slurry thus obtained is held under apredetermined condition. Digestion or maceration is carried out as theleaching treatment under the heating and pressurizing conditions of atemperature of 160° C. or more and 300° C. or less, preferably 180° C.or more and 250° C. or less, and a pressure of 0.65 MPa or more and 10MPa or less, preferably 1 MPa or more and 5 MPa or less, for a holdingtime of 30 minutes or more and 160 minutes or less, preferably 40minutes or more and 120 minutes or less. The reason why the digestion ormaceration is carried out under such heating and pressurizing conditionsas the leaching treatment of rare-earth elements is as follows: Ca andTi existing in the bauxite residue at predetermined ratios exist ascompounds forming crystals having a perovskite-type structure,rare-earth elements such as Nd and Dy, which are highly useful, areincorporated in such crystals having a perovskite-type structure, andhence the digestion or maceration is carried out under such heating andpressurizing conditions to allow the crystals having a perovskite-typestructure to dissolve, thereby causing the rare-earth elements to leach.

Herein, when the digestion or maceration operation is carried out at atemperature of less than 160° C., even if the pressure and the holdingtime are set to necessary and proper conditions, it is difficult tocause rare-earth elements to leach sufficiently, and it is difficult tocause 70 mass % or more of the rare-earth elements contained in thebauxite residue to leach. In contrast, when the temperature is more than250° C., the leaching ratio of rare-earth elements shows an almostsaturated state. Further, heating at more than 300° C. causes anincrease in the amount of necessary heat, degradation of a pressurevessel, an increase in cost, and the like. Meanwhile, when the digestionor maceration operation is carried out under a pressure of less than0.65 MPa, even if the temperature and the holding time are set tonecessary and proper conditions, it is difficult to cause rare-earthelements to leach sufficiently, and it is difficult to cause 70 mass %or more of the rare-earth elements contained in the bauxite residue toleach. In contrast, the digestion or maceration operation carried outunder a pressure of more than 10 MPa results in applying anunnecessarily high pressure, causing degradation of a pressure vessel,an increase in cost, and the like. Further, when the digestion ormaceration operation is carried out for a holding time of less than 30minutes, even if the temperature and the pressure are set to necessaryand proper conditions, stable operation is difficult because of suchshort holding time, the leaching ratio of rare-earth elements cannot bestabilized, and hence it is difficult to cause 70 mass % or more of therare-earth elements contained in the bauxite residue to leach. Incontrast, when the holding time is more than 160 minutes, the leachingratio of rare-earth elements shows an almost saturated state.

In the the present invention, in the leaching treatment (digestion ormaceration) carried out under such heating and pressurizing conditions,rare-earth elements contained in the bauxite residue, in particular,rare-earth elements including Nd and Dy leach together with Ca, andhence it is desirable to carry out the digestion or maceration by using,as an index, the leaching ratio of Ca, which is more abundant than therare-earth elements, and it is desired to carry out the digestion ormaceration until the leaching ratio of Ca exceeds 90 mass %. Carryingout the digestion or maceration until the leaching ratio of Ca exceeds90 mass % enables more than 70 mass % of the rare-earth elementscontained in the bauxite residue to leach certainly.

The slurry after the leaching treatment is then subjected tosolid-liquid separation by means selected from, for example, filtration,centrifugal separation, and decantation, and a leachate includingrare-earth elements together with Ca is recovered. It is preferred thatthe solid residue yielded by the solid-liquid separation be washed withwashing water so that the leachate attached to the solid residue may betransferred into the washing water and recovered, and both the recoveredleachate and the leachate previously yielded by the solid-liquidseparation of the slurry after the leaching treatment be used as aleachate to be treated in the subsequent separation step. When theamount of the washing water to be used for washing the solid residue istoo small, the leachate attached to the solid residue cannot berecovered sufficiently. In contrast, when the amount of the washingwater to be used is too large, a larger burden is applied in thesubsequent separation step. Thus, the ratio of the washing water (L) tothe solid residue (S), that is, the liquid-solid ratio (L/S), desirablyfalls within the range of 2 to 10 in ordinary cases.

The above-mentioned leaching step was described by taking as an examplethe case where a raw material for leaching is a bauxite residue which isa solid residue obtained after aluminum hydroxide was caused to dissolvefrom bauxite by a Bayer process. The raw material for leaching to beused in the leaching step has only to contain rare-earth elements andcontain Ca at a ratio of 4 to 15 mass % and Ti at a ratio of 2 to 13mass %. The raw material for leaching is not particularly limited to thebauxite residue.

The leachate yielded by the above-mentioned leaching step is thentransferred to the separation step of separating and recoveringrare-earth elements.

In the separation step of separating rare-earth elements from theleachate, there is used, as a separation method, an oxalateprecipitation method, a hydroxide precipitation method, or a solventextraction method.

In the present invention, in which the dissolution amounts of Fe and Tiare small, the leachate can be directly treated by an oxalateprecipitation method or a solvent extraction method. However, when thedissolution amount of Al or Fe is large, and the amounts of chemicalsused in the solvent extraction method or the oxalate precipitationmethod increase. Therefore, it is preferred to decrease the amount ofthe leachate through pretreatment in order to reduce the cost.

A method of the pretreatment is exemplified as follows. Because the pHvalue of the leachate yielded in the leaching step usually falls withinthe range of 1 to 3, a pH adjuster is first added to the leachate so asto adjust the pH value thereof to 4 to 6, and hydroxides of Fe and Alprecipitated by this pH adjustment are removed by solid-liquidseparation. The pH adjuster to be used for this purpose is notparticularly limited, and sodium hydroxide, potassium hydroxide, calciumhydroxide, ammonia, or the like is suitably used.

When the pH adjustment of the leachate is performed, an oxidizing agentis desirably added as required, thereby oxidizing Fe²⁺ ions into Fe³⁺ions in the leachate. With this, insoluble Fe(OH)₃ is stabilized, whichfacilitates the separation and removal of Fe. It is possible to usesuitably, as the oxidizing agent, for example, hydrogen peroxide,perchloric acid, permanganic acid, hypochlorous acid, or the like. Whenhydrogen peroxide is used as the oxidizing agent, the concentration ofthe oxidizing agent influences only the liquid-solid ratio, and hence aproper concentration can be selected in consideration of the ease ofhandling and the cost. When the raw material for leaching is the bauxiteresidue, in both the case of using a 30-mass % hydrogen peroxidesolution and the case of using a 70-mass % perchloric acid aqueoussolution, the addition amount of the oxidizing agent is preferably 0.1to 0.5 equivalent weight with respect to the amount of an Fe componentin the bauxite residue.

In the hydroxide precipitation method, in order to separate rare-earthelements as their hydroxides, a pH adjuster is further added to theleachate yielded in the above-mentioned leaching step or a liquidyielded by adjusting the pH of the leachate to cause Fe and Al toprecipitate as their hydroxides, followed by solid-liquid separation,thereby adjusting the pH value to 7 or more, Ca and the rare-earthelements are caused to precipitate as their hydroxides, and thehydroxides of Ca and the rare-earth elements are subjected tosolid-liquid separation and recovered as a crude recovered product. ThepH adjuster is preferably sodium hydroxide, potassium hydroxide, calciumhydroxide, ammonia, or the like, and Ca and the rare-earth elements areprecipitated as their hydroxides. The precipitated hydroxides aresubjected to solid-liquid separation, thereby recovering the hydroxidesof the rare-earth elements. Alternatively, it is preferred that, for thepurpose of reducing the concentration of Al, which is an impurity, asodium hydroxide solution be added to the precipitated hydroxides of therare-earth elements at 5 or more equivalent weights with respect to theAl, thereby causing the Al component to dissolve as aluminate ions andremoving the Al component.

In the oxalate precipitation method, oxalic acid is added to theleachate yielded in the above-mentioned leaching step or a liquidyielded by adjusting the pH of the leachate to cause Fe and Al toprecipitate as their hydroxides, followed by solid-liquid separation, at1.3 to 6 equivalent weights with respect to the total number of moles ofthe rare-earth elements existing in the leachate or the liquid, yieldinginsoluble rare-earth oxalates, and solid-liquid separation is thenperformed, thereby recovering crude rare-earth compounds (cruderecovered product) as rare-earth oxalate compounds.

When crude rare-earth compounds (crude recovered product) are recoveredby a solvent extraction method from the leachate yielded in theabove-mentioned leaching step or a liquid yielded by adjusting the pH ofthe leachate to cause Fe and Al to precipitate as their hydroxides,followed by solid-liquid separation, the solvent extraction method maybe performed by a known method. It is possible to use suitably anextractant prepared by diluting an ester such as a phosphoric acid ester(DEHPA or EHPA), a phosphonic acid ester (PC88A), or a phosphinic acidester (Cyanex 272 or Cyanex 30) with a solvent such as an aliphatichydrocarbon such as hexane, which is a non-polar organic solvent, anaromatic hydrocarbon such as benzene or toluene, an alcohol such asoctanol, or kerosene, which is a petroleum fraction.

It is also preferred to carry out the recovery of a crude recoveredproduct by a solvent extraction method through two or more stages. Whena crude recovered product is recovered by the solvent extraction methodthrough two or more stages, rare-earth elements can be separated intoeach element.

When the solid residue (bauxite residue) remaining after aluminumhydroxide is caused to dissolve from bauxite by a Bayer process is usedas the raw material for leaching and crude rare-earth compounds (cruderecovered product) are recovered by a solvent extraction method from theleachate yielded in the above-mentioned leaching step, it is preferredthat the pH of the leachate be initially adjusted to 2.5 to 3.5, theresultant precipitate be removed, and solvent extraction be performed orthe pH of the leachate be re-adjusted to 1.2 to 2.5, followed by solventextraction. When the pH is adjusted and the precipitate is removed asdescribed above, it is possible to prevent the occurrence of an emulsionor a suspension (hereinafter referred to as “emulsion”) produced, forexample, between the organic phase and aqueous phase at the time of thesolvent extraction. When the emulsion occurs, the resultant precipitatecan be removed by filtration. It is not preferred that the pH of theaqueous phase be less than 1.2 at the time of solvent extraction becausethe recovery ratio of rare-earth elements lowers.

It is also suitable to add a bauxite residue to perform such pHadjustment as described above. When pH adjustment is performed byaddition of a bauxite residue, the amount of alkaline chemicals used canbe suppressed, and, because the bauxite residue is produced as aby-product in a Bayer process for producing aluminum from bauxite, thecost can be reduced as a result. Further, when pH adjustment isperformed by addition of a bauxite residue, rare-earth elementscontained in the added bauxite residue dissolve in the leachate, andhence the acid aqueous solution used in the leaching treatment can beeffectively used, and the rare-earth elements that leach from the addedbauxite residue can be recovered. Moreover, in this case, Ca and Ticoprecipitate with Fe, the concentrations of these elements in theleachate lower, and the rare-earth elements can be efficiently recoveredas a result.

Further, in such case, it is preferred that DEHPA (chemical name:bis(2-ethylhexyl) hydrogen phosphate) be used as an extractant anddiluted with a solvent so as to have a concentration of 0.1 to 1.5 Mbecause the extraction ratio of Al can be kept low, and theconcentration of rare-earth elements separated and recovered can beincreased as a result. The extraction time is preferably 5 minutes orless, more preferably 0.5 to 3 minutes. When the extraction time is 0.5to 3 minutes, the extraction ratio of Al can be kept low, and theconcentration of rare-earth elements separated and recovered can beincreased as a result. When the extraction time is more than 5 minutes,the extraction ratio of Al becomes high, and the concentration ofrare-earth elements separated and recovered reduces as a result.

When DEHPA is used as an extractant, it is also suitable thatpre-extraction be preliminarily performed by using PC88A (chemical name:mono-2-ethylhexyl 2-ethylhexyl phosphonate), tributyl phosphate, ornaphthenic acid as a pre-extractant. When such pre-extraction isperformed, the concentrations of elements such as Fe, Sc, and Ticontained in the leachate can be reduced, and rare-earth elements can beefficiently separated and recovered as a result. In this case, Sc isseparated into the pre-extracted organic phase, but, when backextraction is performed by using an alkaline aqueous solution having apH of 7.5 or more as a back extractant, Sc can be recovered as a solidhydroxide from the pre-extracted organic phase. In this case, Fe and Tihave already been removed, and hence pH adjustment is not required whenrare-earth elements are extracted by using DEHPA as an extractant. Inthis case, however, emulsion sometimes occurs between the organic phaseand aqueous phase at the time of solvent extraction. When the emulsionoccurs, the resultant precipitate can be removed by filtration.

When the back extraction is performed, it is preferred to use a 2 N to 8N aqueous solution of hydrochloric acid or an aqueous solution ofsulfuric acid having a concentration of 30 to 70 mass % as the backextractant.

When the 2 N to 8 N aqueous solution of hydrochloric acid is used as theback extractant, the back extraction time is preferably 5 minutes orless, more preferably 0.5 to 3 minutes. When the back extraction time is0.5 to 3 minutes, the extraction ratio of Al can be kept low, and theconcentration of rare-earth elements separated and recovered can beincreased as a result. When the back extraction time is more than 5minutes, the extraction ratio of Al becomes high, and the concentrationof rare-earth elements separated and recovered reduces as a result.

On the other hand, when the aqueous solution of sulfuric acid having aconcentration of 30 to 70 mass % is used as the back extractant,rare-earth elements are precipitated as solid sulfates, and thus can beextremely reduced in volume. The back extraction time is preferably 5minutes or less, more preferably 0.5 to 3 minutes. When the backextraction time is 0.5 to 3 minutes, the extraction ratio of Al can bekept low, and the concentration of rare-earth elements separated andrecovered can be increased as a result. When the back extraction time ismore than 5 minutes, the extraction ratio of Al becomes high, and theconcentration of rare-earth elements separated and recovered reduces asa result. The rare-earth elements precipitated as solid sulfates can berecovered by performing solid-liquid separation. Note that, after therare-earth elements are recovered, the resultant organic phase can besubjected to back extraction for 120 minutes or more by using an aqueoussolution of sulfuric acid having a concentration of 30 to 70 mass as aback extractant, thereby recovering Al in the organic phase as aluminumsulfate.

When back extraction of a used extractant is performed by using a 2 N to8 N aqueous solution of hydrochloric acid or an alkaline aqueoussolution as a back extractant, Sc, Ti, and Th, which accumulate in theused extractant, can be reduced, and the resultant used extractant canbe reused as a recycled extractant.

When the separation step of rare-earth elements is performed, it isdesired that the separation of the crude recovered product into eachelement be carried out by a solvent extraction method involving using anextractant prepared by diluting an ester selected from phosphoric acidesters, phosphoric acid esters, phosphinic acid esters, thiophosphinicacid esters, and mixtures of these esters and tributyl phosphate and/ortrioctylphosphine oxide with a solvent selected from aliphatichydrocarbons such as hexane, aromatic hydrocarbons such as benzene andtoluene, and kerosene, which is a petroleum fraction.

It is preferred to carry out the separation carried out by such solventextraction method by a countercurrent multistage solvent extractionmethod.

In the separation step of the leachate in the present invention, in thecase of the hydroxide precipitation method, as described above, the pHvalue of the leachate is first adjusted to 4 to 6, hydroxides of Fe andAl precipitated by this pH adjustment are removed by solid-liquidseparation, a pH adjuster is then further added to adjust the pH valueto 7 or more, and the precipitated hydroxides of Ca and rare-earthelements are separated by solid-liquid separation, thereby recovering acrude recovered product. Further, in the case of the oxalate method,oxalic acid is added to the leachate directly or to a liquid yielded byadjusting the pH of the leachate to cause Fe and Al to precipitate astheir hydroxides, followed by solid-liquid separation, as in thehydroxide precipitation method, rare-earth elements are caused toprecipitate as oxalates, the oxalates are recovered as oxalate compoundsof the rare-earth elements, the oxalate compounds are treated withcaustic soda, yielding hydroxides of the rare-earth elements, and thehydroxides are recovered as a crude recovered product, or the oxalatecompounds of the rare-earth elements are calcined, yielding oxides ofthe rare-earth elements, and the oxides are recovered as a cruderecovered product. The crude recovered product is dissolved inhydrochloric acid or nitric acid, followed by solvent extraction byusing an extractant, and there is an advantage in that the amount of anexpensive extractant to be used in the solvent extraction can be reducedas much as possible.

EXAMPLES

The method of recovering rare-earth elements according to the presentinvention is hereinafter specifically described by way of Examples andComparative Examples each using a bauxite residue as a raw material forleaching, but the present invention is not limited by Examples andComparative Examples below.

Examples 1 to 8 and Comparative Examples 1 to 5

There was used, as a raw material for leaching, a bauxite residuecontaining, in a solid component (S) obtained by drying the bauxiteresidue under the drying conditions of 110° C. and 2 hours, 29.8 mass %of Fe, 7.9 mass % of Al, 5.8 mass % of Ca, 2.1 mass % of Na, 3.5 mass %of Ti, 2.5 mass % of Si, and 0.24 mass % of rare-earth elementsincluding Y with an atomic number of 39 and La to Lu with atomic numbersof 57 to 71. About 0.1 kg of the bauxite residue was loaded into apressure vessel, followed by addition of water, yielding a slurry. Afterthat, an aqueous solution of hydrochloric acid or nitric acid was addedso as to attain each of the liquid-solid ratios (L/S) and initial pHvalues shown in Table 2, followed by mixing, preparing each slurrycontaining the bauxite residue.

Next, the slurry was heated and pressurized so that the temperature andpressure in the pressure vessel reached each value shown in Table 2, andwas held for each time period shown in Table 2. After that, the slurrywas filtrated to perform solid-liquid separation under ordinarytemperature and ordinary pressure, and a leachate was recovered.Further, a solid residue obtained after the solid-liquid separation waswashed with 400 cm³ of washing water for 0.1 kg of the solid residue,and the washing water used for the washing and the leachate were usedtogether as a leachate to be treated in the subsequent separation step.The pH value of the leachate was measured to determine the pH of theleachate yielded in the leaching step.

The leachate thus yielded in the leaching step of each of Examples 1 to8 and Comparative Examples 1 to 5 was used to carry out inductivelycoupled plasma-atomic emission spectroscopy (ICP-AES) analysis.Measurement was performed on the content of each of the elements Y, Nd,Dy, Ca, Al, Si, Ti, and Fe in the leachate, and the leaching ratio ofeach element was calculated. Table 2 collectively shows the leachingconditions and results thereof.

TABLE 2 Example 1 2 3 4 5 6 7 8 Use amount (kg) of bauxite residue 0.1020.108 0.107 0.100 0.099 0.111 0.102 0.103 Kind of acid in leaching stepHNO₃ HCl HNO₃ HNO₃ HNO₃ HCl HNO₃ HCl Liquid-solid ratio in slurry 7.57.1 4.2 4.2 4.2 7.5 4.1 7.1 PH Initial stage <0.1 <0.1 <0.1 0.1 1.0 1.12.4 1.6 After leaching 2.4 1.9 1.8 2.5 2.6 2.7 2.9 2.8 LeachingTemperature ° C. 225 230 225 250 180 200 225 200 conditions Pressure MPa2.6 2.8 2.6 4.0 1.0 1.6 2.6 1.6 Time Minutes 60 40 120 120 90 140 120140 Leaching Y 89 81 86 87 77 83 89 81 ratio Nd 78 82 78 80 73 82 80 78(mass %) Dy 86 76 81 82 72 79 79 76 Ca 98 95 98 100 93 98 98 94 Al 63 5559 38 24 26 20 14 Si 7 11 5 7 4 8 16 20 Ti 0.2 0.0 0.1 0.0 0.1 0.1 0.10.1 Fe 0.2 3.0 0.1 0.0 0.0 1.0 0.0 0.8 Comparative Example 1 2 3 4 5 6Use amount (kg) of bauxite residue 0.147 0.101 0.100 0.100 0.102 0.099Kind of acid in leaching step HCl H₂SO₄ H₃PO₄ HNO₃ HClO₄ HNO₃ + HCl(1:1) Liquid-solid ratio in slurry 5.3 8.6 4.3 3.7 7.4 5.3 pH Initialstage 0.2 1.0 1.1 3.0 0.6 <0.0 After leaching 2.3 1.5 2.0 3.3 2.5 <0.0Leaching Temperature ° C. 150 50 225 225 200 100 conditions Pressure MPa0.5 0.1 2.6 2.6 1.6 0.1 Time Minutes 180 30 120 120 140 60 Leaching Y 2114 1 55 67 30 ratio Nd 27 15 1 51 64 50 (mass %) Dy 18 10 1 48 59 29 Ca83 50 14 87 93 68 Al 52 41 15 15 33 99 Si 30 100 7 28 11 1 Ti 0.0 1.60.2 0.2 0.1 37 Fe 0.7 1.2 0.3 0.1 0.1 100

As evident from the results shown in Table 2, in the leachate yielded inthe leaching step of each of Examples 1 to 8, 70 mass or more of therare-earth elements contained in the bauxite residue used as a rawmaterial for leaching were able to be caused to leach. On the otherhand, in each of Comparative Example 1, in which the leachingtemperature was 150° C., Comparative Example 2, in which an aqueoussolution of sulfuric acid was used as an acid aqueous solution, theliquid-solid ratio (L/S) was 8.6, and the leaching temperature was 50°C., Comparative Example 3, in which an aqueous solution of phosphoricacid was used as an acid aqueous solution, Comparative Example 4, inwhich the initial pH value was 3.0, Comparative Example 5, in which anaqueous solution of hypochlorous acid was used as an acid aqueoussolution, and Comparative Example 6, in which the leaching temperaturewas 100° C., 70 mass %; or more of the rare-earth elements contained inthe bauxite residue were unable to be caused to leach.

Examples 9 to 13 and Comparative Examples 6 to 8

Leaching of rare-earth elements was carried out in the same manner asthat in each of Examples 1 to 8 described above, except that eachoxidizing agent shown in Table 3 was added in each acid aqueous solutionused in each leaching step at each equivalent weight shown in Table 3with respect to the Fe content in the bauxite residue. Measurement wasperformed on the content of each of the elements Y, Nd, Dy, Ca, Al, Si,Ti, and Fe in the resultant leachate, and the leaching ratio of eachelement was calculated. Table 3 collectively shows the leachingconditions and results thereof.

TABLE 3 Example Comparative Example 9 10 11 12 13 6 7 8 Use amount (kg)of bauxite residue 0.103 0.101 0.109 0.103 0.105 0.115 0.100 0.116 Kindof acid in leaching step HCl HNO₃ HNO₃ HCl HCl HCl H₂SO₄ HNO₃Liquid-solid ratio in slurry 7.6 7.7 7.2 4.2 7.4 6.9 6.6 6.8 OxidizingKind H₂O₂ H₂O₂ H₂O₂ HClO₄ H₂O₂ H₂O₂ H₂O₂ H₂O₂ agent Addition amount (*1)0.38 0.38 0.35 0.13 0.50 0.34 0.39 0.33 pH Initial stage <0.1 <0.1 <0.10.2 1.8 <0.1 <0.1 <0.1 After leaching 1.5 1.7 1.8 1.5 2.9 2.0 2.3 1.5Leaching Temperature ° C. 200 200 230 200 230 150 200 150 conditionsPressure MPa 1.6 1.6 2.8 1.6 2.8 0.5 1.6 0.5 Time Minutes 140 140 140110 140 130 140 180 Leaching Y 87 90 88 82 84 23 19 32 ratio Nd 81 85 8573 80 30 10 38 (mass %) Dy 80 82 81 78 78 21 13 30 Ca 96 94 94 92 96 8748 82 Al 65 77 71 28 58 53 100 65 Si 9 8 8 5 6 29 9 33 Ti 0.3 0.1 0.40.0 0.2 0.1 0.3 0.1 Fe 2.2 0.2 0.3 0.1 1.5 0.5 0.5 0.8 (*1) Equivalentweight with respect to Fe content in bauxite residue

As evident from the results shown in Table 3, in the leachate yielded inthe leaching step of each of Examples 9 to 13, 70 mass % or more of therare-earth elements contained in the bauxite residue used as a rawmaterial for leaching were able to be caused to leach. On the otherhand, in each of Comparative Examples 6 and 8, in which the leachingtemperature was 150° C., and in Comparative Example 7, in which anaqueous solution of sulfuric acid was used as an acid aqueous solution,70 mass % or more of the rare-earth elements contained in the bauxiteresidue were unable to be caused to leach.

Example 14

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a solvent extraction method, the removalof impurity elements and the concentration of rare-earth elements. Inthe solvent extraction method, first, the pH of the leachate wasinitially adjusted to 3.0, the resultant precipitate was removed, andthe pH was adjusted to 1.5. After that, there was used an extractantprepared by diluting DEHPA with kerosene to a concentration of 0.8 M,and the leachate and the extractant were brought into contact with eachother at a liquid ratio of 1:1 under stirring for 3 minutes. Then, themixture was subjected to liquid-liquid separation into an extractedorganic phase and an aqueous phase after completion of extraction(aqueous phase after extraction).

TABLE 4 Sample HNO₃ leachate pH 2.0 Component (ppm) Y 65.6 La 21.5 Pr5.8 Nd 26.4 Dy 6.6 Ca 2,504 Al 2,978 Si 182 Ti 4.9 Fe 9.7

A 6 N aqueous solution of hydrochloric acid was used as a backextractant, and the extracted organic phase and the back extractant werebrought into contact with each other at a liquid ratio of 1:1 understirring for 3 minutes. Then, the mixture was again subjected toliquid-liquid separation into an organic phase after completion of backextraction (organic phase after back extraction) and a back-extractedaqueous phase. As a result, rare-earth elements in the extracted organicphase were transferred into the back-extracted aqueous phase, and wereseparated and recovered.

When a 0.02 N aqueous solution of hydrochloric acid is used as a backextractant, the organic phase after back extraction and the backextractant are brought into contact with each other at a liquid ratio of1:1 under stirring for 3 minutes, and then the mixture is subjected toliquid-liquid separation, followed by purification, the resultant liquidcan be reused cyclically as an extractant prepared by diluting DEHPAwith kerosene to a concentration of 0.8 M.

Table 5 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Examples 15 to 18

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered under the same conditions asthose in Example 14, except that, in the same method as that in Example14, the time of contact between the leachate and the extractant was setto 0.5 minute, 1 minute, 5 minutes, and 10 minutes.

Table 5 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Examples 19 to 23

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered under the same conditions asthose in Example 14, except that, in the same method as that in Example14, the time of contact between the extracted organic phase and the backextractant was set to 0.5 minute, 1 minute, 5 minutes, 10 minutes, and15 minutes.

Table 5 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Example 24

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a solvent extraction method, the removalof impurity elements and the concentration of rare-earth elements. Inthe solvent extraction method, first, the pH of the leachate wasinitially adjusted to 1.75. After that, there was used an extractantprepared by diluting DEHPA with kerosene to a concentration of 0.8 M,and the leachate and the extractant were brought into contact with eachother at a liquid ratio of 1:1 under stirring for 3 minutes. Then, themixture was subjected to liquid-liquid separation into an extractedorganic phase and an aqueous phase after extraction. Emulsion occurredbetween the organic phase and the aqueous phase at the time of thesolvent extraction, but the emulsion was separated into the organicphase side at the time of the liquid-liquid separation and was thenremoved by filtrating the organic phase with a filter.

A 6 N aqueous solution of hydrochloric acid was used as a backextractant, and the extracted organic phase and the back extractant werebrought into contact with each other at a liquid ratio of 1:1 understirring for 3 minutes. Then, the mixture was again subjected toliquid-liquid separation into an organic phase after back extraction anda back-extracted aqueous phase. As a result, rare-earth elements weretransferred from the extracted organic phase into the back-extractedaqueous phase, and were separated and recovered.

When a 0.02 N aqueous solution of hydrochloric acid is used as a backextractant, the organic phase after back extraction and the backextractant are brought into contact with each other at a liquid ratio of1:1 under stirring for 3 minutes, and then the mixture is subjected toliquid-liquid separation, followed by purification, the resultant liquidcan be reused cyclically as an extractant prepared by diluting DEHPAwith kerosene to a concentration of 0.8 M.

Table 5 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Example 25

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered by the same implementationmethod as that in Example 14 and under the same conditions as those inExample 14, except that pH adjustment was performed by adding the samebauxite residue as that used in Example 4 instead of adding an aqueoussolution of sodium hydroxide. In this case, the amount of the addedbauxite residue was 0.115 kg with respect to 0.1 kg of the bauxiteresidue used as a raw material for leaching.

Table 5 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method. Note that, whenthe recovery ratios were calculated, the rare-earth elements containedin the bauxite residue used for the pH adjustment were taken intoconsideration, and hence recovery ratios with respect to 2.15 times theamount of the bauxite residue used as a raw material for leaching areshown.

TABLE 5 Example 14 15 16 17 18 19 Extraction time (minute(s)) 3 0.5 1 510 3 Back extraction time (minute(s)) 3 3 3 3 3 0.5 Recovery ratio Y 8790 88 84 87 43 (mass %) La 87 85 90 81 76 73 Pr 94 98 93 92 90 93 Nd 9698 96 94 92 94 Dy 100 100 100 100 100 88 Ca 39 53 44 33 29 41 Al 1 1 1 22 0 Si 0 0 0 0 0 0 Ti 4 4 5 3 3 3 Fe 32 19 29 35 36 29 Example 20 21 2223 24 25 Extraction time (minute(s)) 3 3 3 3 3 3 Back extraction time(minute(s)) 1 5 10 15 3 3 Recovery ratio Y 76 88 89 89 78 70 (mass %) La77 77 77 77 71 63 Pr 95 95 95 95 87 64 Nd 96 96 96 96 89 63 Dy 100 100100 100 90 77 Ca 43 43 41 44 41 14 Al 1 2 3 4 1 1 Si 0 0 0 0 0 0 Ti 4 44 4 3 0 Fe 28 28 30 30 32 4

It is found, on the basis of the recovery ratios of the rare-earthelements and impurities in Examples 14 to 23 shown in Table 5, that asthe extraction time is shorter, the recovery ratios of the rare-earthelements are higher, that as the back extraction time is longer, therecovery ratios of the rare-earth elements are higher, but even Y, whichshows the lowest recovery ratio, shows a recovery ratio exceeding 75mass % for a back extraction time of 1 minute, and that as both theextraction time and back extraction time are longer, the recovery ratiosof impurities such as Al are higher.

It is found on the basis of the results of Example 24 that, whenemulsion occurs between the organic phase and the aqueous phase at thetime of the solvent extraction, the recovery ratios of the rare-earthelements in Example 24 are slightly lower in comparison to those inExample 14, in which the extraction time and back extraction time arethe same as those in Example 24.

Further, in Example 25, in which pH adjustment was performed by adding abauxite residue, rare-earth elements which dissolved from the bauxiteresidue added at the time of the pH adjustment were also recovered, butthe recovery ratios of the rare-earth elements were not as high as therecovery ratios of the rare-earth elements which were caused to leachfrom the bauxite residue used as a raw material for leaching. Thus, itis found that the recovery ratios in Example 25 are lower than those inExample 14, but Ca and Ti coprecipitate with Fe and the concentrationsof these elements are significantly reduced. In addition, the bauxiteresidue is produced as a by-product in a Bayer process for producingaluminum from bauxite, resulting in a cost reduction.

Example 26

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a solvent extraction method, the removalof impurity elements and the concentration of rare-earth elements. Inthe solvent extraction method, first, the pH of the leachate wasinitially adjusted to 3.0, the resultant precipitate was removed, andthe pH was adjusted to 1.0. After that, there was used an extractantprepared by diluting DEHPA with kerosene to a concentration of 0.8 M,and the leachate and the extractant were brought into contact with eachother at a liquid ratio of 1:1 under stirring for 3 minutes. Then, themixture was subjected to liquid-liquid separation into an extractedorganic phase and an aqueous phase after extraction.

A 6 N aqueous solution of hydrochloric acid was used as a backextractant, and the extracted organic phase and the back extractant werebrought into contact with each other at a liquid ratio of 1:1 understirring for 3 minutes. Then, the mixture was again subjected toliquid-liquid separation into an organic phase after back extraction anda back-extracted aqueous phase. As a result, rare-earth elements weretransferred from the extracted organic phase into the back-extractedaqueous phase, and were separated and recovered.

When a 0.02 N aqueous solution of hydrochloric acid is used as a backextractant, the organic phase after back extraction and the backextractant are brought into contact with each other at a liquid ratio of1:1 under stirring for 3 minutes, and then the mixture is subjected toliquid-liquid separation, followed by purification, the resultant liquidcan be reused cyclically as an extractant prepared by diluting DEHPAwith kerosene to a concentration of 0.8 M.

Table 6 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Examples 27 and 28

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered under the same conditions asthose in Example 26, except that, in the same method as that in Example26, an extractant prepared by diluting DEHPA with kerosene to aconcentration of 1.2 M and an extractant prepared by diluting DEHPA withkerosene to a concentration of 1.5 M were used.

Table 6 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Examples 29 and 30

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered under the same conditions asthose in Example 26, except that, in the same method as that in Example26, the pH of the leachate was initially adjusted to 3.0, the resultantprecipitate was removed, and the pH was again adjusted to 1.5 or 2.0.

Table 6 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Example 31

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a solvent extraction method, the removalof impurity elements and the concentration of rare-earth elements. Inthe solvent extraction method, first, the pH of the leachate wasinitially adjusted to 3.0, the resultant precipitate was removed, andthe pH was again adjusted to 2.0. After that, there was used anextractant prepared by diluting PC88A with kerosene to a concentrationof 0.8 M, and the leachate and the extractant were brought into contactwith each other at a liquid ratio of 1:1 under stirring for 3 minutes.Then, the mixture was subjected to liquid-liquid separation into anextracted organic phase and an aqueous phase after extraction.

A 6 N aqueous solution of hydrochloric acid was used as a backextractant, and the extracted organic phase and the back extractant werebrought into contact with each other at a liquid ratio of 1:1 understirring for 3 minutes. Then, the mixture was again subjected toliquid-liquid separation into an organic phase after back extraction anda back-extracted aqueous phase. As a result, rare-earth elements weretransferred from the extracted organic phase into the back-extractedaqueous phase, and were separated and recovered.

When a 0.02 N aqueous solution of hydrochloric acid is used as a backextractant, the organic phase after back extraction and the backextractant are brought into contact with each other at a liquid ratio of1:1 under stirring for 3 minutes, and then the mixture is subjected toliquid-liquid separation, followed by purification, the resultant liquidcan be reused cyclically as an extractant prepared by diluting PC88Awith kerosene to a concentration of 0.8 M.

Table 6 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Examples 32 to 34

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered under the same conditions asthose in Example 31, except that, in the same method as that in Example31, an extractant prepared by diluting PC88A with kerosene to aconcentration of 0.5 to 1.5 M was used.

Table 6 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

Examples 35 to 37

Rare-earth elements were transferred into the back-extracted aqueousphase, and were separated and recovered under the same conditions asthose in Example 31, except that, in the same method as that in Example31, the pH of the leachate was initially adjusted to 3.0, the resultantprecipitate was removed, and the pH was again adjusted to 1.5 to 3.0.

Table 6 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

TABLE 6 Example 26 27 28 29 30 31 Kind of extractant DEHPA DEHPA DEHPADEHPA DEHPA PC88A Concentration (M) of extractant 0.8 1.2 1.5 0.8 0.80.8 Adjusted value of pH of leachate 1.0 1.0 1.0 1.5 2.0 2.0 Recoveryratio Y 100 93 80 100 96 94 (mass %) La 26 51 64 80 97 5 Pr 76 91 100100 100 39 Nd 83 97 100 100 100 50 Dy 100 100 100 100 100 94 Ca 11 24 3337 55 1 Al 0 0 0 1 1 8 Si 0 0 0 0 0 0 Ti 3 2 1 3 3 0 Fe 9 9 8 22 31 70Example 32 33 34 35 36 37 Kind of extractant PC88A PC88A PC88A PC88APC88A PC88A Concentration (M) of extractant 0.5 1.2 1.5 0.8 0.8 0.8Adjusted value of pH of leachate 2.0 2.0 2.0 1.5 2.5 3.0 Recovery ratioY 97 94 94 88 97 100 (mass %) La 2 12 18 1 9 10 Pr 18 56 65 11 56 63 Nd25 69 77 14 68 76 Dy 97 97 91 87 100 100 Ca 1 2 2 1 1 1 Al 7 9 8 4 11 13Si 0 0 0 0 0 0 Ti 0 0 0 0 1 0 Fe 82 61 48 65 79 73

It is found, on the basis of the recovery ratios of the rare-earthelements and impurities in Examples 26 to 37 shown in Table 6, that theuse of DEHPA shows higher recovery ratios of the rare-earth elements butlower recovery ratios of Al than the use of PC88A, that as the pH of theleachate is higher in both the case of using DEHPA as an extractant andthe case of using PC88A as an extractant, the recovery ratios of boththe rare-earth elements and Al tend to be higher, that when DEHPA isused as an extractant, as the concentration thereof is higher, therecovery ratios of both the rare-earth elements and Al are higher, andthat when PC88A is used as an extractant, as the concentration thereofis higher, the recovery ratios of the rare-earth elements are higher,but the recovery ratio of Al has its maximum point near theconcentration of 1.2 M.

Examples 38 to 43

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a solvent extraction method includingpre-extraction, the removal of impurity elements and the concentrationof rare-earth elements. In this method, first, the pH of the leachatewas initially adjusted to 3.0, the resultant precipitate was removed,and the pH was again adjusted to 1.0 or 1.25. After that, there was useda pre-extractant prepared by diluting PC88A with kerosene to aconcentration of 0.01 to 0.02 M, and the leachate and the pre-extractantwere brought into contact with each other at a liquid ratio of 1:1 understirring for 3 minutes. Then, the mixture was subjected to liquid-liquidseparation into a pre-extracted organic phase and an aqueous phase afterextraction. Subsequently, there was used an extractant prepared bydiluting DEHPA with kerosene to a concentration of 0.8 M, and therecovered pre-extracted organic phase and the extractant were broughtinto contact with each other at a liquid ratio of 1:1 under stirring for3 minutes. Then, the mixture was subjected to liquid-liquid separationinto an extracted organic phase and an aqueous phase after extraction.

A 6 N aqueous solution of hydrochloric acid was used as a backextractant, and the extracted organic phase and the back extractant werebrought into contact with each other at a liquid ratio of 1:1 understirring for 3 minutes. Then, the mixture was again subjected toliquid-liquid separation into an organic phase after back extraction)and a back-extracted aqueous phase. As a result, rare-earth elementswere transferred from the extracted organic phase into theback-extracted aqueous phase, and were separated and recovered.

When a 0.02 N aqueous solution of hydrochloric acid is used as a backextractant, the organic phase after back extraction and the backextractant are brought into contact with each other at a liquid ratio of10:1 under stirring for 3 minutes, and then the mixture is subjected toliquid-liquid separation, followed by purification, the resultant liquidcan be reused cyclically as an extractant prepared by diluting DEHPAwith kerosene to a concentration of 0.8 M.

Table 7 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

TABLE 7 Example 38 39 40 41 42 43 Concentration (M) 0.01 0.015 0.02 0.010.015 0.02 of extractant Adjusted value of pH 1.0 1.0 1.0 1.25 1.25 1.25of leachate Recovery ratio Y 95 90 94 78 76 75 (mass %) La 24 24 23 4545 43 Pr 69 67 70 70 68 67 Nd 75 78 74 74 72 72 Dy 91 93 91 81 79 77 Ca9 8 10 18 18 18 Al 0 0 0 1 1 1 Si 0 0 0 0 0 0 Ti 2 1 1 1 1 1 Fe 8 8 8 1716 15

It is found, on the basis of the recovery ratios of the rare-earthelements and impurities in Examples 38 to 43 shown in Table 7, that therecovery ratios of the rare-earth elements are kept at almost the samelevel in comparison to those in Example 26, but the recovery ratios ofCa and Ti among the impurities are significantly lowered.

Examples 44 to 52

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a solvent extraction method, the removalof impurity elements and the concentration of rare-earth elements. Inthe solvent extraction method, first, the pH of the leachate wasinitially adjusted to 3.0, the resultant precipitate was removed, andthe pH was again adjusted to 1.0. After that, there was used anextractant prepared by diluting DEHPA with kerosene to a concentrationof 0.8 M, and the leachate and the extractant were brought into contactwith each other at a liquid ratio of 1:1 under stirring for 3 minutes.Then, the mixture was subjected to liquid-liquid separation into anextracted organic phase and an aqueous phase after extraction.

A 50 mass % aqueous solution of sulfuric acid was used as a backextractant, and the extracted organic phase and the back extractant werebrought into contact with each other at a liquid ratio of 1:1 understirring for 1 to 180 minutes. Elements including the rare-earthelements were precipitated as solid sulfates, and hence the solidsulfates containing the rare-earth elements were recovered bysolid-liquid separation.

When a 0.02 N aqueous solution of hydrochloric acid is used as a backextractant, the organic phase after back extraction and the backextractant are brought into contact with each other at a liquid ratio of1:1 under stirring for 3 minutes, and then the mixture is subjected toliquid-liquid separation, followed by purification, the resultant liquidcan be reused cyclically as an extractant prepared by diluting DEHPAwith kerosene to a concentration of 0.8 M.

Table 8 shows the recovery ratios of the rare-earth elements andimpurities recovered by this solvent extraction method.

TABLE 8 Example 44 45 46 47 48 Back extraction time (minute(s)) 1 3 5 3060 Recovery ratio Y 44 91 95 95 95 (mass %) La 21 23 23 23 23 Pr 57 6263 63 63 Nd 64 69 69 69 69 Dy 80 100 100 100 100 Ca 10 11 11 11 11 Al0.0 0.0 0.0 0.1 0.1 Si 0 0 0 0 0 Ti 3 0 0 0 0 Fe 0 0 0 0 0 Example 49 5051 52 Back extraction time (minute(s)) 90 120 150 180 Recovery ratio Y95 95 95 95 (mass %) La 23 23 23 23 Pr 63 61 60 60 Nd 69 69 69 69 Dy 100100 100 99 Ca 11 11 11 11 Al 0.1 0.2 0.2 0.2 Si 0 0 0 0 Ti 0 0 0 0 Fe 00 0 0

It is found, on the basis of the recovery ratios of the rare-earthelements and impurities in Examples 44 to 52 shown in Table 8, that Feand Ti are hardly recovered, but the rare-earth elements can be eachrecovered at a high recovery ratio, and that as the back extraction timeis longer, the recovery ratio of Al is higher, but when the backextraction time is 5 minutes or less, the recovery ratio of Al can bekept at a low value of less than 0.1%.

Example 53

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform the removal of impurity elements and theconcentration of rare-earth elements by the two-stage solvent extractionmethod illustrated in FIG. 1. The details are hereinafter described withreference to FIG. 1.

The two-stage solvent extraction method was performed as follows. First,in an extraction operation A (Ext. A), the pH of a leachate (1) wasadjusted to 2.0, an extractant prepared by diluting DEHPA with hexane toa concentration of 0.02 M was then used, the leachate (1) and theextractant were brought into contact with each other at a liquid ratioof 1:1 under stirring for 3 minutes, and then the mixture was subjectedto liquid-liquid separation into an extracted organic phase A (2) and anaqueous phase after extraction A (3).

In this case, Y and Dy are contained in the extracted organic phase A(2), and the rare-earth elements ranging from La to Nd are contained inthe aqueous phase after extraction A (3).

For the extracted organic phase A (2), in a back extraction operation A(R-Ext. A), a 0.2 N aqueous solution of hydrochloric acid was used as aback extractant, the extracted organic phase A (2) and the backextractant were brought into contact with each other at a liquid ratioof 1:1 under stirring for 3 minutes, the mixture was then subjected toliquid-liquid separation again into an organic phase after backextraction A (4) and a back-extracted aqueous phase A (5), and Y and Dywere separated from the extracted organic phase A (2) into theback-extracted aqueous phase A (5).

For the organic phase after back extraction A (4), in a purificationoperation (P), a 2 N aqueous solution of hydrochloric acid is used as aback extractant, the organic phase after back extraction A (4) and theback extractant are brought into contact with each other at a liquidratio of 1:1 under stirring for 3 minutes, and then the mixture issubjected to liquid-liquid separation, followed by purification. Then,the resultant liquid can be reused cyclically as an extractant preparedby diluting DEHPA with hexane to a concentration of 0.02 M, and the usedback extractant is discarded as a waste liquid (W).

Further, for the above-mentioned back-extracted aqueous phase A (5)containing Y and Dy separated from the extracted organic phase] A (2),in an extraction B (Ext. B), an extractant prepared by diluting DEHPAwith hexane to a concentration of 0.02 M was used, the back-extractedaqueous phase A (5) and the extractant were brought into contact witheach other at a liquid ratio of 1:1 under stirring for 5 minutes, andthen the mixture was subjected to liquid-liquid separation into anextracted organic phase B (6) and an aqueous phase after extraction B(7), discarding the aqueous phase after extraction B (7) as a wasteliquid (8).

For the above-mentioned extracted organic phase B (6), in a backextraction operation B (R-Ext. B), a 2 N aqueous solution ofhydrochloric acid was used as a back extractant, the extracted organicphase B (6) and the back extractant were brought into contact with eachother at a liquid ratio of 1:1 under stirring for 5 minutes, the mixturewas then subjected to liquid-liquid separation into an organic phaseafter back extraction B (9) and a back-extracted aqueous phase B (10),and Y and Dy were separated by being transferred from the extractedorganic phase B (6) to the above-mentioned back-extracted aqueous phaseB (10) and were recovered as a recovery No. 1 (11).

When the organic phase after back extraction B (9) is subjected to thesame treatment as that in the above-mentioned purification operation (P)(not shown), the resultant liquid can be reused cyclically as anextractant prepared by diluting DEHPA with hexane to a concentration of0.02 M.

On the other hand, after the pH of the above-mentioned aqueous phaseafter extraction A (3) was adjusted to 2, there was performed anextraction operation C (Ext. C), in which an extractant prepared bydiluting DEHPA with hexane to a concentration of 0.8 M was used, theaqueous phase after extract ion A (3) and the extractant were broughtinto contact with each other at a liquid ratio of 1:1 under stirring for3 minutes, and then the mixture was subjected to liquid-liquidseparation into an extracted organic phase C (12) and an aqueous phaseafter extraction C (13), discarding the aqueous phase after extraction C(13) as a waste liquid (14).

For the above-mentioned extracted organic phase C (12), in a backextraction operation C (R-Ext. C), a 0.1 N aqueous solution ofhydrochloric acid was used as a back extractant, the extracted organicphase C (12) and the back extractant were brought into contact with eachother at a liquid ratio of 1:1 under stirring for 5 minutes, and themixture was then subjected to liquid-liquid separation into an organicphase after back extraction C (15) and a back-extracted aqueous phase C(16). As a result, Ca was removed from the extracted organic phase C(12) and the back-extracted aqueous phase C (16) containing Ca wasdiscarded as a waste liquid (17).

Then, for the above-mentioned organic phase after back extraction C(15), in a back extraction operation D (R-Ext. D), a 2 N aqueoussolution of hydrochloric acid was used as a back extractant, the organicphase after back extraction C (15) and the back extractant were broughtinto contact with each other at a liquid ratio of 1:1 under stirring for5 minutes, the mixture was then subjected to liquid-liquid separationinto an organic phase after back extraction D (18) and a back-extractedaqueous phase D (19), and the rare-earth elements ranging from La to Ndwere separated from the organic phase after back extraction C (15) intothe back-extracted aqueous phase D (19). Oxalic acid was added to theback-extracted aqueous phase D (19), thereby causing rare-earth oxalatesto precipitate, and the rare-earth elements ranging from La to Nd wererecovered as a recovery No. 2 (20).

When the organic phase after back extraction D (18) is subjected to thesame treatment as that in the above-mentioned purification operation (P)(not shown), the resultant liquid can be reused cyclically as anextractant prepared by diluting DEHPA with hexane to a concentration of0.8 M.

Table 9 shows the recovery ratios of the rare-earth elements recoveredby this two-stage solvent extraction method and the concentrations ofthe impurities.

Example 54

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by an oxalate precipitation method, theremoval of impurity elements and the concentration of rare-earthelements. In the oxalate precipitation method, oxalic acid was added tothe leachate of Example 4 at about 1.5 chemical equivalent weights withrespect to the rare-earth ions contained in the leachate, only therare-earth elements were caused to precipitate as oxalates, andsolid-liquid separation was performed, thereby recovering the rare-earthoxalates.

Table 9 shows the recovery ratios of the rare-earth elements recoveredby the oxalate precipitation method and the concentrations of theimpurities.

Example 55

The leachate yielded in Example 4 and having the composition shown inTable 4 was used to perform, by a hydroxide precipitation method, theremoval of impurity elements and the concentration of rare-earthelements. In the hydroxide precipitation method, first, the pH of theleachate of Example 4 was adjusted to pH 4.5 at which the solubility ofAl ions and the solubility of Fe ions were small and the solubility ofrare-earth ions was large, thereby causing Al and Fe to precipitate ashydroxides, and the precipitated hydroxides of Al and Fe were removed bysolid-liquid separation. After that, a caustic soda solution was furtheradded to the resultant liquid, increasing the pH thereof to 11,rare-earth ions were caused to precipitate as hydroxides, andsolid-liquid separation was performed, thereby recovering the rare-earthhydroxides.

Table 9 shows the recovery ratios of the rare-earth elements recoveredby the hydroxide precipitation method and the concentrations of theimpurities.

TABLE 9 Impurity separation method Example 53 Two-stage solvent Example54 Example 55 extraction method Oxalate Hydroxide Recovery Recoveryprecipitation precipitation No. 1 No. 2 method method Recovery Y 92.00.2 95.1 85.8 ratio La 1.3 40.1 98.4 89.7 (mass%) Pr 0.0 91.7 96.2 84.3Nd 0.0 96.9 95.9 84.5 Dy 97.1 2.1 93.5 82.4 Ca 0.2 1.0 8.4 5.4 Al 0.80.0 0.06 0.89 Si 0.1 0.0 0.17 2.99 Ti 23.7 0.0 1.31 0.00 Fe 21.3 0.20.10 0.30

The invention claimed is:
 1. A method of recovering rare-earth elements,comprising: a leaching step including preparing a slurry by adding waterto a raw material for leaching which contains rare-earth elements,followed by mixing, further adding an acid aqueous solution to theslurry, followed by mixing, to adjust a pH of the slurry, performingleaching treatment in which the rare-earth elements in the raw materialfor leaching are transferred into the acid aqueous solution while theslurry is held under a predetermined condition, and then subjecting theslurry, which is obtained after the leaching treatment to solid-liquidseparation, thereby yielding a leachate containing the rare-earthelements; and a separation step of separating and recovering therare-earth elements from the leachate yielded in the leaching step,wherein: the raw material for leaching comprises Ca as CaO at a ratio offrom 4 to 15 mass % and Ti as TiO₂ at a ratio of from 2 to 13 mass % ina solid component (S) obtained by drying the raw material for leachingunder drying conditions of 110° C. and 2 hours; the acid aqueoussolution comprises an acid aqueous solution which contains hydrochloricacid and/or nitric acid and adjusts the pH to from 0 to 2.7; and theleaching treatment performed in the leaching step is digestion ormaceration which is performed under heating and pressurizing conditionsof a temperature of from 160 to 300° C. and a pressure of from 0.65 to10 MPa, and the rare-earth elements in the raw material for leaching arecaused to leach together with Ca in the leaching step.
 2. A method ofrecovering rare-earth elements according to claim 1, wherein thedigestion or maceration in the leaching step is performed until adissolution ratio of Ca contained in the raw material for leachingreaches 90 mass % or more.
 3. A method of recovering rare-earth elementsaccording to claim 1, wherein the raw material for leaching comprises abauxite residue produced as a by-product in a Bayer process forcollecting an aluminum component from bauxite by using an aqueoussolution of sodium hydroxide.
 4. A method of recovering rare-earthelements according to claim 3, wherein the bauxite residue comprisesrare-earth elements as oxides thereof at a ratio of from 500 to 10,000ppm in a solid component (S) obtained by drying the bauxite residueunder drying conditions of 110° C. and 2 hours.
 5. A method ofrecovering rare-earth elements according to claim 3, wherein the slurryobtained by adding the acid aqueous solution to the bauxite residue hasa liquid-solid ratio (L/S) of a liquid component (L) and a solidcomponent (S) of from 2 to 10 and a pH value of from 0 to 2.7.
 6. Amethod of recovering rare-earth elements according to claim 3, furthercomprising adding an oxidizing agent into the slurry prepared by addingthe acid aqueous solution to the bauxite residue, at a ratio of from 0.1to 1 equivalent weight with respect to an Fe component in the bauxiteresidue.
 7. A method of recovering rare-earth elements according toclaim 6, wherein the oxidizing agent added into the slurry is a hydrogenperoxide solution or a perchloric acid aqueous solution.
 8. A method ofrecovering rare-earth elements according to claim 1, further comprising:adjusting a pH of the leachate yielded in the leaching step to from 4 to6 by adding a pH adjuster to the leachate; removing, by solid-liquidseparation, hydroxides of Fe and Al precipitated owing to the adjustingof the pH; and subjecting a resultant liquid to the separation step. 9.A method of recovering rare-earth elements according to claim 8, whereinthe adjusting of the pH to from 4 to 6 by adding the pH adjuster to theleachate comprises adding, to the leachate, an oxidizing agent selectedfrom hydrogen peroxide, perchloric acid, permanganic acid, andhypochlorous acid to oxidize Fe²⁺ ions into Fe³⁺ ions in the leachate.10. A method of recovering rare-earth elements according to claim 1,wherein the separation step of the rare-earth elements comprises: addinga pH adjuster to the leachate yielded in the leaching step or to aliquid yielded by adjusting a pH of the leachate to cause Fe and Al toprecipitate as hydroxides thereof, followed by solid-liquid separationto adjust a pH of the leachate or the liquid to 7 or more; andseparating hydroxides of Ca and the rare-earth elements, which arecaused to precipitate owing to the pH adjustment, by solid-liquidseparation to recover the hydroxides as a crude recovered product.
 11. Amethod of recovering rare-earth elements according to claim 10, furthercomprising separating the crude recovered product into each element bydissolving the crude recovered product in an acid aqueous solution andcarrying out a solvent extraction method which uses an extractantprepared by diluting an ester selected from phosphoric acid esters,phosphonic acid esters, phosphinic acid esters, thiophosphinic acidesters, and mixtures of these esters and tributyl phosphate and/ortrioctylphosphine oxide with a solvent selected from hexane, benzene,toluene, and kerosene.
 12. A method of recovering rare-earth elementsaccording to claim 11, wherein the separation of the crude recoveredproduct into the each element by the solvent extraction method comprisesa countercurrent multistage solvent extraction method.
 13. A method ofrecovering rare-earth elements according to claim 1, wherein theseparation step of the rare-earth elements comprises: adding oxalic acidto the leachate yielded in the leaching step or to a liquid yielded byadjusting a pH of the leachate to cause Fe and Al to precipitate ashydroxides thereof, followed by solid-liquid separation, at a ratio of achemical equivalent weight equal to or more than that of the rare-earthelements existing therein, to cause the rare-earth elements toprecipitate as oxalates thereof; and separating the oxalates bysolid-liquid separation to recover the rare-earth elements as a cruderecovered product.
 14. A method of recovering rare-earth elementsaccording to claim 1, wherein the separation step of the rare-earthelements comprises: adding an extractant to the leachate yielded in theleaching step or to a liquid yielded by adjusting a pH of the leachateto cause Fe and Al to precipitate as hydroxides thereof, followed bysolid-liquid separation, the extractant being prepared by diluting anester selected from phosphoric acid esters, phosphonic acid esters,phosphinic acid esters, thiophosphinic acid esters, and mixtures ofthese esters and tributyl phosphate and/or trioctylphosphine oxide witha solvent selected from hexane, benzene, toluene, octanol, and kerosene;and separating and recovering the rare-earth elements by a solventextraction method.
 15. A method of recovering rare-earth elementsaccording to claim 14, further comprising, prior to the separation stepby the solvent extraction method, removing emulsion which occurs duringthe adjusting of the pH of the leachate in advance by filtration.
 16. Amethod of recovering rare-earth elements according to claim 14, furthercomprising: prior to the separation step by the solvent extractionmethod, adjusting the pH of the leachate to from 2.5 to 3.5; andremoving the resultant precipitate.
 17. A method of recoveringrare-earth elements according to claim 16, wherein the adjusting of thepH performed prior to the separation step by the solvent extractionmethod comprises adding a bauxite residue.
 18. A method of recoveringrare-earth elements according to claim 14, wherein the extractant usedin the solvent extraction method comprises bis(2-ethylhexyl)hydrogenphosphate).
 19. A method of recovering rare-earth elements according toclaim 18, wherein the bis(2-ethylhexyl)hydrogen phosphate) serving asthe extractant used in the solvent extraction method has a concentrationof from 0.1 to 1.5 M.
 20. A method of recovering rare-earth elementsaccording to claim 18, further comprising performing pre-extraction ofthe leachate by using mono-2-ethylhexyl 2-ethylhexyl phosphonate,tributyl phosphate, or naphthenic acid as a pre-extractant to separateand remove Fe, Sc, and Ti from the leachate, prior to the solventextraction method which uses the bis(2-ethylhexyl)hydrogen phosphate) asthe extractant.
 21. A method of recovering rare-earth elements accordingto claim 14, wherein an extraction time in the solvent extraction methodis 5 minutes or less.
 22. A method of recovering rare-earth elementsaccording to claim 21, wherein the extraction time in the solventextraction method is from 0.5 to 3 minutes.
 23. A method of recoveringrare-earth elements according to claim 14, wherein the solventextraction method uses a 2 N to 8 N aqueous solution of hydrochloricacid as a back extractant and a back extraction time in the solventextraction method is 5 minutes or less.
 24. A method of recoveringrare-earth elements according to claim 23, wherein the back extractiontime in the solvent extraction method is from 0.5 to 3 minutes.
 25. Amethod of recovering rare-earth elements according to claim 14, whereinthe solvent extraction method uses an aqueous solution of sulfuric acidhaving a concentration of from 30 to 70 mass % as a back extractant andthe rare-earth elements are recovered as solid sulfates.
 26. A method ofrecovering rare-earth elements according to claim 25, wherein a backextraction time in the solvent extraction method is 5 minutes or less.27. A method of recovering rare-earth elements according to claim 14,wherein the solvent extraction method comprises: subjecting a usedextractant to back extraction by using a 2 N to 8 N aqueous solution ofhydrochloric acid or an alkaline aqueous solution as a back extractantto reduce Sc, Ti, and Th accumulating in the used extractant; and usingthe resultant used extractant as a recycled extractant.